Electric cables, in particular for heavy-duty applications and/or for mobile installations, such as mobile harbour cranes, ship-to-shore container cranes, ship un-loaders, spreaders, and mining and tunnelling equipment, are specifically designed to withstand harsh environment conditions and high mechanical stresses, such as tensile forces and torques. As a further example of cables for heavy-duty applications, down well pump cables for supplying current to submersible electricity pump systems in deep wells are usually installed in physically restricted areas and in hostile environments, often being in contact with corrosive well fluids. Typically, the above cables are designed to be robust and flexible. Within the present description, we will in general refer to heavy-duty cables, when referring to cables for heavy-duty applications and in particular, but not exclusively, for mobile installations.
An example of heavy-duty electric cable is provided in DE 3934718, which describes an armoured trailing cable for shearer loaders in mines.
WO 01/78086 discloses an electric cable in particular for use in a pick-up system such as a crane or shelving system. The cable comprises a core, which includes first conductors, completely surrounded by and embedded within a first stress-bearing matrix. At least one further layer is disposed about the first stress-bearing matrix and has at least one further conductor in the further layer which is completely surrounded by and embedded within a second stress-bearing matrix. The stress-bearing matrices in the cable are said to allow the distribution of stress throughout the cable and thus to substantially reduce the corkscrew effect.
Tensile loads and twisting in a mobile cable may result from forced guidance of the cable during the winding and unwinding phases around reels or from collection of the cable within baskets (e.g., for spreader cables). Winding and unwinding phases are typically discontinuous and often abrupt, for example when caused by a horizontal movement of a crane, thereby imposing significant dynamic tensile loads on the cable, and thus on the individual conductors within the cable. In addition, other systems for cable movement, such as pulley systems and tender systems, generally involve high tensile loads on the cable during operation.
Excessive elongation of the cable can cause the tensile loads to be transferred to the electrical conductors with consequent damage of the latter. Excessive and/or prolonged tensile loads may result in a permanent elongation of the cable, which would shorten the life of the cable.
U.S. Pat. No. 5,767,956 describes the use of backscattering Brillouin light to provide a monitoring device that is capable of observing, in real time, whether an optical fibre is normal or on the verge of fracture. The device uses optical time domain reflectometry (OTDR) to monitor a stimulated Brillouin scattering light by utilizing one of optical fibre cores in an optical cable. No hint is provided about the use in an electric cable.
WO 08/073,033 describes a system for monitoring the bending and strain of a power cable connected to a moving offshore platform by measuring the strain in optical fibres attached to or incorporated into the power cable. A bend in the power cable will give rise to a strain in the optical fibre and this strain will change the optical properties of the fibre. The change in optical properties can be measured by means of optical time domain reflectometer (OTDR) or optical frequency domain reflectometer (OFDR).
This document does not face the problem of protecting the strain sensor from external mechanical stresses in order to avoid damage of the sensor and to ensure long-term reliability of the measurements. On the contrary, the application states that there exists a risk that the optical fibres embedded or attached to the cable might be damaged and thus it is suggested to equip the cable with redundant fibres. Furthermore, there is no mention of the problem of strain transfer between the fibres and the cable to be measured. As possible location for the fibre, the interstices between armouring wires are mentioned.
US 2004/0258373 describes a composite cable, which can be embedded in buildings, umbilical or pipelines, comprising optical means for monitoring temperature and strain. The cable comprises an outer protective sheath and optical means for monitoring temperature and strain, said optical means being within said outer protective sheath and comprising: a first tube including at least a first optical fibre in order to monitor the temperature, said first optical fibre being loose in said first tube and comprising at least one reflecting section called Bragg grating, at least a second optical fibre including at least one Bragg grating in order to monitor the strain, said cable being characterized in that said second optical fibre is outside said first tube, said optical means further comprising means for tightening said second optical fibre.
The Applicant has noted that this document does not disclose an electrical cable with an integrated strain optical fibre sensor, or with an integrated temperature sensor. The disclosed cable is a composite cable that can include power cables, however separated from the fibres for strain and temperature monitoring.
EP 0203249 discloses a medium-voltage (from 6 to 60 kV) power cable that includes at least one temperature and/or tension sensor optical fibre.
The Applicant has observed that the disclosed strain sensor optical fibre integrated in the cable can be significantly affected by bending of the cable depending on the position of the optical fibre within the cable and/or the amount of bending of the cable, in particular when exceeding a certain value.
Chen Xihao and Huang Junhua, in Strain Transfer Capability of Strain Sensing Optical Fiber Cable and its Measurement Method, published in the Proceedings of the 57th International Wire & Cable Symposium (2008), pages 424-428, analyse different structures of sensing optical fibre cables (i.e. cables to be used for sensing the strain of an associated device or system). The tightness of layers within the strain sensing optical fibre cable is said to be of great importance and can be described by the strip force between the cable layers and the strain transfer capability, i.e., the maximum strain that can be transferred from the cable outer layer to the inner fibre without reduction.
This document does not mention any application of the disclosed sensing optical fibre cables to cable monitoring of power or electric cables.
WO 07/107,693 discloses a fibre optic cable including a strain transfer member, a central optical fibre disposed through the strain transfer member, and a tight jacket mechanically coupling the central optical fibre and the strain transfer member. Strain experienced by the strain transfer member is transferred to the central optical fibre via the tight jacket.
The document does not face the problem of an electric cable with a fibre optic sensor.
The Applicant has been faced with the problem of how to realise an electric cable, in particular suitable for heavy-duty applications and more particularly for mobile installations, which would allow controlling, and preferably real-time monitoring, of the tensile strain to which the cable is subjected during operation while ensuring long-term reliability of the measurements.
In particular, the Applicant has tackled the problem of carrying out measurements in an electric cable by a sensor which remain substantially unaffected by external mechanical stresses others than tensile strain imposed on the cable.